sd_bus_track_new() creates a new bus peer tracking object. The object
is allocated for the specified bus, and returned in the *ret
parameter. After use, the object should be freed again by dropping
the acquired reference with sd_bus_track_unref() (see below). A bus
peer tracking object may be used to keep track of peers on a specific
IPC bus, for cases where peers are making use of one or more local
objects, in order to control the lifecycle of the local objects and
ensure they stay around as long as the peers needing them are around,
and unreferenced (and possibly destroyed) as soon as all relevant
peers have vanished. Each bus peer tracking object may be used to
track zero, one or more peers add a time. References to specific bus
peers are added via sd_bus_track_add_name(3) or
sd_bus_track_add_sender(). They may be dropped again via
sd_bus_track_remove_name() and sd_bus_track_remove_sender().
Alternatively, references on peers are removed automatically when
they disconnect from the bus. If non-NULL the handler may specify a
function that is invoked whenever the last reference is dropped,
regardless whether the reference is dropped explicitly via
sd_bus_track_remove_name() or implicitly because the peer
disconnected from the bus. The final argument userdata may be used to
attach a generic user data pointer to the object. This pointer is
passed to the handler callback when it is invoked.
sd_bus_track_ref() creates a new reference to a bus peer tracking
object. This object will not be destroyed until sd_bus_track_unref()
has been called as many times plus once more. Once the reference
count has dropped to zero, the specified object cannot be used
anymore, further calls to sd_bus_track_ref() or sd_bus_track_unref()
on the same object are illegal.
sd_bus_track_unref() destroys a reference to a bus peer tracking
object.
sd_bus_track_unrefp() is similar to sd_bus_track_unref() but takes a
pointer to a pointer to an sd_bus_track object. This call is useful
in conjunction with GCC's and LLVM's Clean-up Variable Attribute[1].
Note that this function is defined as inline function.
sd_bus_track_ref(), sd_bus_track_unref() and sd_bus_track_unrefp()
execute no operation if the passed in bus peer tracking object is
NULL.
Bus peer tracking objects may exist in two modes: by default they
operate in non-recursive mode, but may optionally be switched into
recursive mode. If operating in the default non-recursive mode a peer
is either tracked or not tracked. In this mode invoking
sd_bus_track_add_name() multiple times in a row for the same peer is
fully equivalent to calling it just once, as the call adds the peer
to the set of tracked peers if necessary, and executes no operation
if the peer is already being tracked. A single invocation of
sd_bus_track_remove_name() removes the reference on the peer again,
regardless how many times sd_bus_track_add_name() was called before.
If operating in recursive mode, the number of times
sd_bus_track_add_name() is invoked for the same peer name is counted
and sd_bus_track_remove_name() must be called the same number of
times before the peer is not tracked anymore, with the exception when
the tracked peer vanishes from the bus, in which case the count is
irrelevant and the tracking of the specific peer is immediately
removed. sd_bus_track_get_recursive() may be used to determine
whether the bus peer tracking object is operating in recursive mode.
sd_bus_track_set_recursive() may be used to enable or disable
recursive mode. By default a bus peer tracking object operates in
non-recursive mode, and sd_bus_track_get_recursive() for a newly
allocated object hence returns a value equal to zero. Use
sd_bus_track_set_recursive() to enable recursive mode, right after
allocation. It takes a boolean argument to enable or disable
recursive mode. Note that tracking objects for which
sd_bus_track_add_name() was already invoked at least once (and which
hence track already one or more peers) may not be switched from
recursive to non-recursive mode anymore.
sd_bus_track_get_bus() returns the bus object the bus peer tracking
object belongs to. It returns the bus object initially passed to
sd_bus_track_new() when the object was allocated.
sd_bus_track_get_userdata() returns the generic user data pointer set
on the bus peer tracking object at the time of creation using
sd_bus_track_new() or at a later time, using
sd_bus_track_set_userdata().

Returned errors may indicate the following problems:
-EBUSY
Bus peers have already been added to the bus peer tracking object
and sd_bus_track_set_recursive() was called to change tracking
mode.
-EINVAL
Specified parameter is invalid (NULL in case of output
parameters).
-ENOMEM
Memory allocation failed.

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systemd 234 SD_BUS_TRACK_NEW(3)